The present invention relates to the magnet arts. It finds particular application in conjunction with magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated, that the invention will also find application in conjunction with magnetic resonance spectroscopy and other applications which use strong, controlled magnetic fields.
Heretofore, open or vertical field magnetic resonance imaging systems have typically included a pair of circular pole pieces disposed parallel to each other on opposite sides of an examination region. The pole pieces included various ferrous, magnetic, and other structures for shaping the magnetic field in the examination region. The pole pieces also supported radio field and gradient magnetic field coils.
Typically, cylindrical ferrous elements extended from the back sides of the pole face away from the examination region. The cylindrical ferrous elements commonly connected with ferrous flux return paths reduce fringe fields and electromagnetic drive coil requirements. Of course, magnetic fields will return through the air and non-ferrous structures, with a greater expenditure of energy.
Magnetic coils were commonly placed around the cylindrical ferrous members and adjacent the pole pieces to generate a magnetic field that flows between the pole pieces through the examination region and back around through a ferrous or non-ferrous return path. Resistive and superconducting magnets have been utilized. Electromagnetic coils have also been placed along the return path, primarily when the magnets have concentrated return paths as are found in C-magnets or H-magnets.
Magnetic field uniformity in the region of interest is a significant concern in magnetic resonance imaging applications. Circular symmetry of the pole pieces and adjacent rose ring ferrous structure has been used extensively as a tool for achieving and promoting this uniformity.
The prior open magnetic resonance imaging systems have tended to have relatively small imaging regions. Placement of the electromagnetic coils for generating the magnetic field was usually close to the poles. This symmetrical circular arrangement supported a uniform and symmetrical magnetic field distribution in the imaging region. Power dissipation in resistive coils and saturation properties of the ferrous core were additional engineering considerations. Unfortunately, the electromagnetic coils often occupy valuable space close to the imaging region and introduce thermal management problems near the poles.
The present invention overcomes the above-referenced problems and others.